In high noise environment, in order to protect audition and perform normal communication, the noise reduction headphone is widely used.
During the process of development and production of active noise reduction headphones, the noise reduction amount of the headphone must be tested to determine whether the headphone is qualified. The main work frequency band of the feedback active noise reduction headphone is generally in the range of 20 Hz-4 kHz. In the currently known test solutions, the headphone is worn on a simulation human head or a similar device, and a set of external noise sources are used to generate noise with enough large sound pressure level and enough low frequency at a certain distance. The noise reduction switch of the headphone is switched so as to obtain the difference between the noise picked up by the simulation human head or the similar device before and after the noise reduction function is activated, as the noise reduction amount of the noise reduction headphone. Here, the larger the power of the external noise source is, the larger the sound pressure level thereof will be. The sound pressure level is irrelevant with the frequency.
FIG. 1 is a schematic view of the structure of the existing feedback active noise reduction headphone. As shown in FIG. 1, the feedback active noise reduction headphone 1 is provided with a noise reduction microphone 4 in front of a receiver 2 of the headphone. The noise reduction microphone 4 picks up the noise within the headphone shell. After the noise reduction circuit 3 performs the amplification and filtering processing, the receiver 2 emits a control signal with amplitude equal and phase opposite to the noise collected by the noise reduction microphone 4. The remainder noise after the control signal is superposed and counteracted with the noise within the headphone shell is further picked up, processed and counteracted by the noise reduction microphone 4. This process is repeated until the remainder noise in headphone shell is stable. The process of the feedback noise reduction processing is a negative feedback process, which is briefly illustrated in FIG. 2.
FIG. 2 is a schematic view of mathematical model of the noise reduction principle of the existing feedback active noise reduction headphone. Referring to FIG. 2, symbol G represents the transfer function from the receiver 2 to the noise reduction microphone 4, symbol H represents the transfer function of the noise reduction circuit 3, symbol z represents the noise within the headphone shell, and symbol c represents the remainder noise, then:
  S  =            c      z        =          1              1        -        GH            wherein, symbol S represents the noise reduction amount of the feedback system. From the principle of the feedback active noise reduction headphone, as long as G and H remain invariable, the feedback system will stably reduce the signal picked up by the noise reduction microphone by S times.
In order to measure the noise reduction amount of the feedback active noise reduction headphone accurately, it is required that the noise at the noise reduction microphone is at least S times larger than the background noise of environmental when the noise reduction function is deactivated. By this way, when the noise reduction function is activated, the remainder noise will not be less than the background noise, whereby the difference value between them two can really represent the noise reduction amount of the headphone. Thus it can be seen that a high-power external noise source is required in order to realize effective test. In particular, in the test on a product line, the background noise in the production plant is generally high, and is concentrically distributed in low frequency range, and thus the requirement on low frequency noise of the noise source is higher, which increases test cost and brings large noise pollution.
Therefore, a critical difficulty in implementing the above test solution is the external noise source requires large enough power and low enough frequency, and such a test system may cause noise pollution to the surrounding environment.
In order to avoid noise pollution, the existing test is usually performed in a shielding room. However, in this way, the demand condition of the test is further increased, i.e., the complexity of the test is increased.